They can be good... depending on the technology. And this is the issue. There isn't "one" 3d tech, there is a few kinds and they all are a little different.

I've used 3D tv's to watch movies here in Canada on display at shops and they are bad. The picture looks flickery and it gives me a headache quickly. When I was in South Korea, there was a 3D tv on display outside a store and decided to try it. It was amazing. Vivid picture, no flickering, no headache, it really felt like it was there right to the point that it looked like I could touch the flower petals and butterflies shown (it was some K-Pop star music video, very over the top girly...) And that 3D tv made me see why people talk about 3D tv's because that was amazing... but I don't know the tech used in it and so no dice.